Re: [Xen-devel] [PATCH] libxl: fix dom0 autoballooning with Xen 4.8

[Jim Fehlig <jfehlig@xxxxxxxx>]

On 02/02/2017 04:42 AM, Wei Liu wrote:
> I saw this mail but didn't realise it required my input, sorry.
I suppose it didn't and I was shamelessly fishing for a review - so you have my 
apologies :-). But the patch does fix an annoying, easily encountered bug which 
I'm eager to see resolved in the next libvirt release.
> On Wed, Feb 01, 2017 at 04:00:54PM -0700, Jim Fehlig wrote:
> > Jim Fehlig wrote:
> > > xen.git commit 57f8b13c changed several of the libxl memory
> > > get/set functions to take 64 bit parameters. The libvirt
> > > libxl driver still uses uint32_t variables for these various
> > > parameters, which is particularly problematic for the
> > > libxl_set_memory_target() function.
> > >
> > > When dom0 autoballooning is enabled, libvirt (like xl) determines
> > > the memory needed to start a domain and the memory available. If
> > > memory available is less than memory needed, dom0 is ballooned
> > > down by passing a negative value to libxl_set_memory_target()
> > > 'target_memkb' parameter. Prior to xen.git commit 57f8b13c,
> > > 'target_memkb' was an int32_t. Subtracting a larger uint32 from
> > > a smaller uint32 and assigning it to int32 resulted in a negative
> > > number. After commit 57f8b13c, the same subtraction is widened
> > > to a int64, resulting in a large positive number. The simple
> > > fix taken by this patch is to assign the difference of the
> > > uint32 values to a temporary int32 variable, which is then
> > > passed to 'target_memkb' parameter of libxl_set_memory_target().
> > >
> > > Note that it is undesirable to change libvirt to use 64 bit
> > > variables since it requires setting LIBXL_API_VERSION to 0x040800.
> > > Currently libvirt supports LIBXL_API_VERSION >= 0x040400,
> > > essentially Xen >= 4.4.
> > Any comments on this patch? xen-devel is already in cc, but I'll add Wei (one of
> > the Xen tools maintainers) and Juergen (author of commit 57f8b13c) for some
> > additional exposure :-).
> >
> > Currently, "out-of-the-box" Xen >= 4.8 + libvirt setup (dom0 autoballooning
> > enabled) is broken without this fix.
> >
> > Regards,
> > Jim
> >
> > > Signed-off-by: Jim Fehlig <jfehlig@xxxxxxxx>
> > > ---
> > >  src/libxl/libxl_domain.c | 4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/src/libxl/libxl_domain.c b/src/libxl/libxl_domain.c
> > > index a5314b0..ed73cd2 100644
> > > --- a/src/libxl/libxl_domain.c
> > > +++ b/src/libxl/libxl_domain.c
> > > @@ -909,6 +909,7 @@ libxlDomainFreeMem(libxl_ctx *ctx, libxl_domain_config 
> > > *d_config)
> > >  {
> > >      uint32_t needed_mem;
> > >      uint32_t free_mem;
> > > +    int32_t target_mem;
> > >      int tries = 3;
> > >      int wait_secs = 10;
> > >
> > > @@ -922,7 +923,8 @@ libxlDomainFreeMem(libxl_ctx *ctx, libxl_domain_config 
> > > *d_config)
> > >          if (free_mem >= needed_mem)
> > >              return 0;
> > >
> > > -        if (libxl_set_memory_target(ctx, 0, free_mem - needed_mem,
> > > +        target_mem = free_mem - needed_mem;
> > > +        if (libxl_set_memory_target(ctx, 0, target_mem,
> This should do the trick.
Thanks. There are other ways to skin this cat and if folks prefer one of those I 
can push a followup. But for now, so that we have a fix, I'll push this variant.
Regards,
Jim


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